According to a comment on stackexchange.com[0] "the message was Printed on a Color Laser Printer and photographed (or scanned)... The 'hidden message' is a result of a previous print leaving a tiny bit of toner on the drum which was picked up by the next print."
The newspapers have prewritten obituaries for many famous people including the ex presidents. Perhaps the prime minister office had two prewritten congratulation notes, and the ghost image is just an old error and not a secret leak of a recent error.
Anyway, they should have been more careful.
Embarrassing amateurism level of the UK government.
I thought the entire point of the current UK government was to be amateurish. Having enough of experts, and all that.